What is the half life of potassium-40?

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Potassium-40 has a half-life of 1.25 billion years. Its extremely long half-life makes it valuable as a tool for assigning a date to deposits where it is found. For example, paleontologists use potassium-40 to determine the age of fossils from between 200,000 and 4 billion years ago.

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Potassium degrades into argon according to a fixed time table. After 1.25 billion years, half of the initial potassium deposit is gone, leaving behind argon gas. Since argon is a noble gas, it is very inert and does not form compounds under ordinary conditions; any argon found in a geological deposit comes from decayed potassium. Scientists compare the ratio of argon to potassium to determine the age of geological deposits. Potassium-argon dating is usually done by examining volcanic rock.